Abstract
A global model (T80L18; Triangular Truncation at wave number 80 with 18 vertical layers) and a mesoscale model MM5 (nested at 90 and 30 km resolutions) are integrated for 5 monsoon years 1998-2002. The impact of dynamical downscaling from global to mesoscale in the simulations of Indian summer monsoon rainfall is studied. Comparisons between the global and the mesoscale models show that, though the global model has an edge over the mesoscale model in simulating the all-India mean rainfall closer to the observation, the T80L18 model lacks in simulating the spatial variations in rainfall. The effect of downscaling is better represented in the rainfall variations produced by MM5 both quantitatively and qualitatively over the foothills of the Himalayas and along Nepal to North-eastern India. It is also seen that the mesoscale model is able to represent the dispersion (standard deviation) present in the observed rainfall over India. In the five monsoon seasons, RMSE of mean rainfall (monthly and seasonal) of T80L18 forecasts are mostly lower than that of MM5 forecasts. However, synoptic features like the Somali Jet and Tibetan anticyclone are better represented by MM5. This model has also simulated the regions of convection better than the T80L18 model. However, the MM5 simulations produced an anomalous circulation over the Saudi Arabian region (15-200 N and 45-500 E) in many cases. The mesoscale model simulates better wind fields than the global model in general. Over peninsular India T80L18 model showed higher temperature gradient but, over Central India this model has better temperature field as compared to MM5. Over southern and north-eastern India, the temperature field of T80L18 and MM5 are very similar.
Highlights
Any change or variation in climate and climatic scenarios affecting the south west monsoon over India, directly affects the Indian economy
Since the NCEP reanalysis is considered as the observed truth, we find that the mesoscale model better represents the magnitude of the Somali Jet in comparison to the global model
A global T80 model and a mesoscale model MM5 has been integrated for 5 monsoon years from 1998-2002
Summary
Any change or variation in climate and climatic scenarios affecting the south west monsoon over India, directly affects the Indian economy. As compared to day-to-day and weekly forecast, seasonal forecasting should be more feasible as the effects of shorter time scale events tend to average out to a large extent, revealing the smaller but more persistent influence of the ocean and land surface on the atmosphere. Estimates of potential predictability on seasonal time scales of the Indian monsoon is limited [2]. They [2] have shown that the Indian Monsoon climate is only marginally predictable in monthly time scales as the contribution of the boundary forcing in this region is relatively low and that of the internal dynamics is relatively
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